Excerpts from David Mazieres's message of 2014-04-28 07:29:06 +0200:
> Austin Clements <amdragon@MIT.EDU> writes:
>
> > As for storing this information directly in messages, in general, the
> > notmuch community is opposed to modifying messages.  This causes many
> > problems, and immutable messages are more robust and simplify so many
> > things.  IMAP assumes messages are immutable.  Maildir assumes
> > messages are immutable.  Notmuch new would get dramatically slower if
> > it had to check for messages modifications.  What do you do if you
> > change a tag and there are multiple copies of a message?  What do you
> > do if there are multiple copies and they disagree about the tags?  How
> > do you atomically update the tags stored in a message?  From an
> > engineering standpoint, it's much better to avoid mutable messages.
>
> The speed penalty would be very minor in the common case.  Muchsync
> scans directories (since it has to scan file contents) and the cost to
> compute SHA-1 hashes of modified files is under 50 msec or something in
> the common case.  Extracting tags would be even cheaper.  The reason is
> that A) you only need to scan modified directories, and B) you don't
> need to open the file unless the inode, mtime, or size has changed.
> Originally I was going to implement an optimization to detect renamed
> files and avoid computing SHA-1 again (for the case where maildir flags
> have changed), but in the end this wasn't even worth it because the cost
> is so small.
>
> That said, I agree that the complexity of altering files is not worth
> it.  Especially since most imap servers will not know about this.  Also,
> the question of what do you do with duplicate message IDs (which is
> effectively what you have when the tags disagree) is a more general
> problem still needing a solution, and would be exacerbated by embedding
> important information like tags in the message.
>
> Really what you want is an imap server built on top of the notmuch
> library.  That way you could use notmuch from your desktop and then use
> imap from your phone, and everything would stay perfectly in sync.
> Implementing such a server wouldn't be that hard, but it would help if
> notmuch made the _notmuch_message_get_doc_id and
> _notmuch_directory_get_document_id functions semi-public.  Then the imap
> server could just use docids as uids.  (Plus then muchsync wouldn't have
> to go through gross contortions to get docids information...)

That would be nice, but a solution where the user does not need to run
his own server is in my opinion pretty essential.

- gaute